Field of Invention
[0001] The present invention relates to the catalytic conversion of hydrocarbons.
Background
[0002] Hydrogen is an important element for many chemical processes and is used in many
technical methods. Hydrogen is required, for instance, as fuel for the generation
of electric energy in fuel cells on board motor vehicles. However, because of the
hazards of storing hydrogen, hydrogen is typically not stored on motor vehicles. Instead,
apparatuses and processes have been developed to generate hydrogen on board motor
vehicles from gasoline or diesel fuel in order to form a gas stream that contains
hydrogen.
[0003] In one method, for instance, steam is added to the fuel and this mixture is converted
in what is referred to as a steam reforming process into hydrogen and carbon monoxide.
The hydrogen is used as fuel for a PEM fuel cell. This steam reforming process is
endothermic and requires an additional burner for heating the reforming reactor to
the required reaction temperature of from about 700 to 800°C.
[0004] Alternatively, the reforming process can be conducted auto thermally by adding a
certain amount of oxygen to the mixture of steam and fuel such that first heat is
generated by catalytic partial oxidation (CPO) of the fuel and then causing steam
reforming to commence. According to this process, the reformate comprises about 30
to 40 % by volume of hydrogen and additionally 10 to 15 % by volume of carbon monoxide,
which must be reduced to below 50 ppm by volume by means of subsequent purification
steps (high and low temperature shift steps; HTS and LTS; preferential oxidation PROX)
in order to avoid contamination of the PEM fuel cell.
[0005] A process for auto thermal steam reforming of hydrocarbons is described in US 2002/0009408
A1. This process requires heating of the reaction mixture to a preheating temperature
and then feeding the reaction mixture to a catalytic reactor for performing the auto
thermal steam reforming. In total, conventional auto thermal steam reforming is performed
in a bulky and elaborate apparatus.
[0006] Since hydrogen or a gas enriched in hydrogen may be used in a vehicle for various
purposes, there is a need to develop a more compact apparatus and efficient process
for the generation of hydrogen on board a vehicle. The present invention provides
one solution to this problem.
Summary of Invention
[0007] The present invention is directed to the catalytic conversion of hydrocarbons, and
provides an apparatus for generating hydrocarbons, a method of use of the apparatus,
and a process for generating a gas stream enriched in hydrogen. The resulting gas
stream may then be exposed to apparatuses such as fuel cells, particulate filters,
and NOx storage catalysts.
[0008] The apparatus comprises a catalyst, as well as a device for supplying hydrocarbons
to the catalyst. In the apparatus, the catalyst is arranged in an exhaust pipe through
which the exhaust gases of combustion processes are passed. Additional hydrocarbons
are added to the exhaust stream, and when sufficient reforming conditions are established,
may be used as a source of hydrogen without the introduction of an external ignition
mechanism. Thus, it permits the efficient generation of hydrogen from the exhaust
gases of internal combustion engines, especially from the exhaust gases of lean burning
engines.
[0009] According to one embodiment, the present invention provides an apparatus for the
catalytic conversion of hydrocarbons that comprises:
a) a first exhaust gas pipe, wherein said first exhaust gas pipe is capable of carrying
a first partial exhaust gas stream;
b) a fuel injector and a reforming catalyst located in sequence within said first
exhaust gas pipe; and
c) a second exhaust gas pipe, wherein said second exhaust gas pipe is capable carrying
a second partial exhaust gas stream.
[0010] Preferably, the apparatus of the present invention is operated under lean conditions
in an exhaust system of an internal combustion engine, especially a diesel engine.
[0011] According to a second embodiment, the present invention provides a process for generating
an exhaust gas enriched in hydrogen and carbon monoxide; said process comprises the
steps of:
a) introducing fuel into a lean exhaust gas stream to reduce its normalized air/fuel
ratio to a value of below 1 and conducting this exhaust gas stream over a reforming
catalyst;
b) burning a first portion of the injected fuel on the catalyst to heat the catalyst
to a reforming temperature; and
c) converting a second portion of said injected fuel into a gas comprising carbon
monoxide and hydrogen.
[0012] This process offers a means for generating a gas enriched in hydrogen and carbon
monoxide. The exhaust stream that is used already contains water and possesses a temperature
suitable for burning hydrocarbons on the catalyst. This exhaust gas stream in combination
with the first portion of the injected fuel is used to generate heat, carbon monoxide
and hydrogen by catalytic partial oxidation, that is by burning a first portion of
the injected hydrocarbons at the catalyst with the oxygen contained in the exhaust
gas. With decreasing oxygen content and increasing temperature of the exhaust gas
stream the operating conditions become more favorable for steam reforming of a second
portion of the injected fuel.
[0013] The process offers the advantage of reduced soot formation during partial oxidation
due to the water content of the exhaust gas. Still further, due to the high temperatures,
the system automatically burns off adhering soot with the oxygen contained in the
exhaust gas.
[0014] The present invention is particularly beneficial for generating hydrogen and carbon
monoxide for mobile and relatively small stationary applications. The exhaust gas,
which is enriched in hydrogen and carbon monoxide can be used in various ways. For
example, it is conventional to inject hydrocarbons into the exhaust gas of lean burn
engines to regenerate nitrogen oxides storage catalysts with respect to stored nitrogen
oxides or with respect to poisoning with sulfur compounds. It is also known to inject
additional hydrocarbons in front of an oxidation catalyst followed by a particulate
filter to regenerate the particulate filter with the heat generated by burning the
hydrocarbons on the oxidation catalyst. In all such applications it is advantageous
to replace the hydrocarbons with hydrogen and carbon monoxide generated with the above
process because hydrogen and carbon monoxide are much more reactive species than hydrocarbons.
[0015] Thus, according to the present invention it is contemplated for all applications,
wherever additional hydrocarbons have to be injected into the exhaust gas stream to
perform a certain task, not to use the hydrocarbons directly but to reform them and
use the generated hydrogen and carbon monoxide instead to fulfill the task.
Brief Description of the Figures
[0016]
- Figure 1:
- Schematic representation of a certain embodiment of the apparatus according to the
invention
- Figure 2:
- Possible arrangement of first and second exhaust pipes to one another
- Figure 3:
- Schematic representation of a certain embodiment of the apparatus according to the
invention including a fuel cell
Detailed Description
[0017] The present invention is directed toward the catalytic conversion of hydrocarbons
for generating a gas rich in hydrogen. An apparatus, a method for using the apparatus
and a process are provided.
[0018] This disclosure is not intended to be a primer on catalyst or hydrogen generation
technologies. Basic concepts are known to persons skilled in the art and are not set
forth in detail herein.
[0019] According to one embodiment, the present invention provides an apparatus for the
catalytic conversion of hydrocarbons in order to generate a gas that is rich in hydrogen.
The apparatus comprises a catalyst and a device for supplying hydrocarbons to the
catalyst. The device for supplying the hydrocarbons is referred to herein as a "fuel
injector."
[0020] The apparatus of the present invention may, for example, be located within the exhaust
stream of a diesel engine or other lean burn engines. As persons skilled in the art
are aware, diesel engines are typically operated under lean conditions. Thus, in a
diesel engine, the exhaust gas prior to the addition of hydrocarbons by the fuel injector
will have a normalized lambda value or normalized air/fuel ratio greater than or equal
to approximately one.
[0021] The catalyst is arranged in an exhaust pipe, referred to herein as "a first exhaust
gas pipe," and carries a first exhaust gas stream, that is a first portion of the
exhaust gases of combustion processes. Preferably, under lean operating conditions,
the catalyst is capable of converting hydrocarbons into water and carbon dioxide and
under reforming conditions, able to convert the supplied hydrocarbons at least partially
into carbon monoxide and hydrogen. More preferably, the catalyst is a reforming or
a three-way catalyst.
[0022] The apparatus of the present invention further comprises a second exhaust gas pipe.
The first exhaust gas pipe and second exhaust gas pipe may be physically separate,
forming parallel exhaust streams or the first exhaust gas pipe may be located within
the second exhaust gas pipe, in which case the outer surface of the first exhaust
gas pipe forms the inner surface of the second exhaust gas pipe.
[0023] In the apparatus, one portion of the exhaust gas from the engine travels into the
first exhaust gas pipe, and another portion of the exhaust gas from the engine travels
into the second exhaust gas pipe. A fuel injection mechanism or fuel injector adds
hydrocarbons to a first exhaust gas stream that travels through the first exhaust
gas pipe. Preferably, one adds the hydrocarbons to the first exhaust gas pipe upstream
of the catalyst. The amount of fuel and exhaust gas flow needed is a function of the
flow through the catalyst, the temperature of the catalyst and the H
2- and CO- concentration that is needed in the utilization step, which, for example,
is the step in which the hydrogen and carbon monoxide are used to regenerate a downstream
catalyst.
[0024] Preferably, the apparatus further comprises a temperature probe for measuring the
temperature of the catalyst. The use of the temperature probe permits the automatic
monitoring of when the injected fuel should be added. Thus, the injection may be self-starting.
[0025] The first exhaust gas pipe and the second exhaust gas pipe are part of the total
exhaust gas system and are located downstream of the engine. The first exhaust gas
pipe and the second exhaust gas pipe are structured such that a portion of the exhaust
gas from the engine enters one or the other exhaust gas pipes thereby forming a first
partial exhaust gas stream and a second partial exhaust gas stream, respectively.
[0026] Preferably, the apparatus comprises a means for controlling partial flow of exhaust
gas between said first and second exhaust gas pipes. Such means may be a throttle
to control the amount of exhaust gas entering the first exhaust gas pipe, and more
preferably a separate throttle is used to enable the control of the amount of exhaust
gas to enter the second exhaust gas pipe, as well.
[0027] In a basic embodiment, the first exhaust gas pipe and the second exhaust gas pipe
are located downstream of a motor, and the aforementioned throttles or other mechanisms
for controlling the flow of a volume of gas are present at the beginning of each of
the exhaust gas pipes. Additionally, preferably, within the first exhaust gas pipe
is a means to supply atomization air in connection with a means by which to supply
the hydrocarbons downstream of the throttle for the first exhaust gas pipe. These
hydrocarbons are referred to herein as "injected fuel."
[0028] Downstream of the entry point of the injected fuel, is a reforming zone, in which
the aforementioned reforming catalyst ("the first catalyst" or "catalytic converter")
is located, and where under appropriate conditions, the hydrogen is generated. Further
downstream of the reforming zone, the first exhaust gas stream is joined with the
second exhaust gas stream to form a joined exhaust gas stream. This joined exhaust
gas stream may then, for example, be exposed to a particle filter or a nitrogen storage
catalyst.
[0029] It is particularly advantageous for the first exhaust gas pipe to be located within
the second exhaust gas pipe. Under this sub-embodiment, the first exhaust pipe may
be designed to form a short pipe piece that has an outer surface area and an inner
surface. The short pipe may be positioned within a second exhaust pipe such that the
second partial flow of the exhaust gas is passed along the outer surface area of the
first exhaust pipe. In this embodiment, the heat of the exhaust gas within the second
exhaust gas pipe may be used to heat the outer surface area of the first exhaust gas
pipe, which will heat the inner surface area and the first exhaust gas itself, and
facilitate the establishment of reforming conditions.
[0030] The apparatus of the present invention may be used in many different applications,
including applications in which other catalysts, referred to herein as "second catalysts"
are located downstream of the first catalyst. For example, the exhaust gas of the
first catalyst may be supplied as fuel to the anode of a fuel cell. The anode exhaust
gas may then be joined with second exhaust gas stream.
[0031] In another embodiment, the apparatus may comprise a NOx storage catalyst located
downstream of the first catalyst and the exhaust gas that contains increased levels
of hydrogen may be utilized for engine independent regeneration of the NOx storage
catalyst.
[0032] In still another embodiment, the apparatus may comprise a particulate filter located
downstream of the first catalyst. The exhaust gas, which contains increased amounts
of hydrogen, may be utilized for the engine-independent regeneration of the particulate
filter.
[0033] The apparatus is particularly beneficial for the generation of hydrogen for mobile
and relatively small stationary applications, and may, for example, be used as an
electric power supply unit for a motor vehicle.
[0034] Under a preferred process for operating the apparatus of the present method, hydrogen
may be generated by introducing injected fuel into a lean first exhaust gas stream
upstream of a first catalyst such to reduce the air/fuel ratio to a value of below
1. Both residual unburned and partially burned hydrocarbons that enter the exhaust
stream from the engine, as well as hydrocarbons from the injected fuel may be burned
at the catalyst to heat the catalyst to a reforming temperature. After reforming conditions
have been established, another portion of the hydrocarbons from the injected fuel,
may be converted into a gas comprising carbon monoxide and hydrogen.
[0035] For example, one may use a partial stream of exhaust gases from a diesel engine or
a stationery burner, (a first exhaust gas stream) when the exhaust exceeds a minimum
temperature of, for example, 200 °C. A hydrocarbon-containing fuel, for example, diesel,
gasoline, or liquid gas, may be added to the first exhaust gas stream. Within this
partial stream, addition of air is limited so that the maximum value of the normalized
air/fuel ratio λ, is less than 1 and preferably less than 0.8, and preferably is between
0.5 and 0.6 after the hydrocarbon-containing fuel has been injected. If liquid fuel
is used, then preferably the fuel is metered by a single or two component nozzle,
an ultrasonic atomizer or other comparable means for fuel atomization.
[0036] While the exhaust gas is lean, the unburned or partially burned hydrocarbons from
the engine are converted into water and carbon dioxide while generating heat. This
process will commence between approximately 200 °C and 250 °C. Additional fuel may
be injected into this stream, which until sufficient reformation conditions are established
will also be converted to H
2O and CO
2, generating heat. The portion of the injected fuel that is converted into H
2O and CO
2 may be referred to as a "first portion of injected fuel." However, when sufficiently
rich conditions are generated, the presence of the heat and water establishes steam
reformation conditions and the formation of carbon monoxide and hydrogen.
[0037] The portion of the injected fuel that is converted into CO and H
2 may be referred to as a "second portion of injected fuel." The phrases first portion
of injected fuel and second portion of injected fuel are used for the convenience
of identifying the two different products. The portion of fuel themselves may be supplied
from the same fuel injector in a continuous stream, and whether a portion of the injected
fuel is denoted as part of the first portion or the second portion is dictated by
whether the requisite conditions to convert the hydrocarbons into hydrogen have been
achieved.
[0038] Thus, under normal operating conditions, lean exhaust gas enters both the first exhaust
gas pipe and the second exhaust gas pipe. The injection of the fuel at an appropriate
temperature may be monitored and triggered by the aforementioned temperature probe.
[0039] The formation of hydrogen, which takes place under rich conditions in the catalyst,
is a combination of the steam reforming process and the partial oxidation of the hydrocarbon
in the fuel. Under typical steam reforming processes, an additional water supply is
needed. However, according to the present invention, water (in the form of steam)
is already present in the exhaust gas stream. The water (steam) in the exhaust gas
prevents soot formation.
[0040] The reforming temperature will be reached by the exhaust gas that preferably has
been combined with the first portion of the injected fuel and burned at an appropriate
installation point within the exhaust gas line. This self-starting mechanism avoids
the need for additional hardware like spark plugs, high-voltage equipment and safety
installations.
[0041] The gases that are produced may be used directly for regeneration of particulate
filters or NOx storage catalysts. Further, these gases may be used to operate fuel
cells.
[0042] The invention will now be further explained with the help of the accompanying figures.
[0043] Figure 1 gives a schematic representation of a specific embodiment of the apparatus
according to the invention. The lean exhaust gases of the diesel engine (1) exit the
engine via exhaust pipe (2). Exhaust pipe (2) splits into a first exhaust pipe (3)
and a second exhaust pipe (4). First exhaust pipe (3) carries a first partial exhaust
gas stream over reforming catalyst (5) and is rejoined with a second partial exhaust
gas stream carried by the second exhaust gas pipe. The rejoined exhaust gas stream
is then cleaned in an exhaust gas cleaning unit (9) before the exhaust gas is emitted
into the atmosphere. The exhaust gas cleaning unit (9) may be a single catalyst or
a suitable arrangement of several catalysts and/or a particulate filter. The ultimate
choice of catalyst(s) depends on the degree of exhaust gas cleaning which has to be
achieved.
[0044] This apparatus is used to produce an exhaust gas enriched in carbon monoxide and
hydrogen on demand e.g. to treat the catalysts contained in the cleaning unit. Such
treatment may be initiated to regenerate a particulate filter or a nitrogen oxide
storage catalyst contained in the exhaust gas cleaning unit.
[0045] Arrow (8) in figure 1 symbolizes a means for introducing hydrocarbons, e.g. diesel
fuel, into the first exhaust gas stream in front of reforming catalyst (5). These
hydrocarbons are then converted into carbon monoxide and hydrogen by auto thermal
steam reforming at the reforming catalyst. Any suitable reforming catalyst known in
the art for this task can be used - e.g. the catalysts described in US 2002/0009408
A1 comprising at least one platinum group metal on an oxidic support material selected
from the group consisting of aluminum oxide, silicon dioxide, titanium dioxide or
mixed oxides thereof or zeolites. Most preferably, the catalysts comprise rhodium
and optionally platinum on an active aluminum oxide. Conventional three-way catalysts
can also be used.
[0046] The means for introducing the hydrocarbons into the first exhaust gas stream at (8)
can be any spray nozzles known in the art such as ultrasonic nozzles, single component
nozzles or two-component nozzles which use e.g. air to atomize the diesel fuel.
[0047] Valves (6) and (7) are optional and allow changing the mass flow relation between
the first and second exhaust stream. Thereby it is possible to change the air/fuel
ratio of the rejoined exhaust gas stream from lean to reach values as required to
treat the subsequent exhaust gas cleaning unit (9).
[0048] Figure 2 shows a preferred arrangement of the apparatus of the invention. The first
exhaust gas pipe (3), which comprises the catalyst (5) for auto thermal reforming
of the hydrocarbons introduced via nozzle (8), is arranged within the second exhaust
gas pipe (4). In this apparatus, second exhaust gas pipe (4) forms only a section
of the exhaust gas pipe (2) of the diesel engine. The exhaust gas stream (10) coming
from the manifold of the diesel engine splits into the first exhaust gas stream (11)
and second exhaust gas stream (12' and 12"). The first exhaust gas stream enters the
first exhaust gas pipe (3). After leaving the first exhaust gas pipe the first exhaust
gas stream is rejoined with the second exhaust gas stream to form the rejoined exhaust
gas stream (13).
[0049] The temperature of the exhaust gas stream entering the first exhaust gas pipe depends
on the operating point of the diesel engine and may vary between 100 and 500 °C. In
the embodiment of figure 2, the second exhaust gas stream flows along the jacket surface
of the first exhaust gas pipe and helps to establish and maintain the starting temperature
for auto thermal reforming, which depends on the light off temperature of catalyst
(5) for catalytic partial oxidation. Therefore, introduction of hydrocarbons into
the first exhaust gas stream for performing auto thermal steam reforming, is only
started when the exhaust gas temperature of the diesel engine has exceeded said light
off temperature.
[0050] The reformed exhaust gas stream leaving the first exhaust gas pipe has a temperature
of approximately 600 to 800 °C.
[0051] Figure 3 shows another embodiment of the apparatus of the invention. In this embodiment
a solid oxide fuel cell (14) has been arranged after reforming catalyst (5). The reformed
exhaust gas stream leaving the reforming catalyst contains increased amounts of carbon
monoxide and hydrogen and is directly used as fuel gas for the solid oxide fuel cell.
The direct use of the reformed exhaust gas stream as fuel is possible since a solid
oxide fuel cell has high operating temperatures and is not sensitive to carbon monoxide
poisoning as a polymer electrolyte fuel (PEM) cell. But it is also contemplated to
lie within the scope of this invention to cool and purify the reformed exhaust gas
stream so that it can be used as fuel for a PEM fuel cell. As shown in figure 3, the
off-gas of the fuel cell is rejoined with the second exhaust gas stream and finally
cleaned in exhaust gas cleaning unit (9).
[0052] Having thus described and exemplified the invention with a certain degree of particularity,
it should be appreciated that the claims that follow are not to be so limited but
are to be afforded a scope commensurate with the wording of each element of the claims
and equivalents thereof.
1. An apparatus for auto thermal steam reforming of hydrocarbons, said apparatus comprises:
a) a first exhaust gas pipe, wherein said first exhaust gas pipe is capable of carrying
a first partial exhaust gas stream;
b) a fuel injector and a reforming catalyst located in sequence within said first
exhaust gas pipe; and
c) a second exhaust gas pipe, wherein said second exhaust gas pipe is capable of carrying
a second partial exhaust gas stream.
2. The apparatus according to claim 1, wherein said apparatus further comprises a means
for controlling partial flow of exhaust gas between said first exhaust gas pipe and
said second exhaust gas pipe.
3. The apparatus according to claim 2, wherein downstream of said reforming catalyst,
said first exhaust gas pipe and said second exhaust gas pipe are combined to form
a combined exhaust gas pipe such that said first exhaust gas stream and said second
exhaust gas stream form a joined exhaust gas stream.
4. The apparatus according to claim 3, wherein the combined exhaust gas pipe contains
an exhaust gas cleaning unit for cleaning the joined exhaust gas stream before it
is emitted into the atmosphere.
5. The apparatus according to claim 4, wherein said exhaust gas cleaning unit contains
in sequence a diesel oxidation catalyst and a particulate filter.
6. The apparatus according to claim 4, wherein said exhaust gas cleaning unit contains
a nitrogen oxide storage catalyst.
7. The apparatus according to claim 4, further comprising a solid oxide fuel cell located
within said first exhaust gas pipe and downstream of said reforming catalyst.
8. An automotive engine comprising a diesel engine and the apparatus of claim 1.
9. A process for generating an exhaust gas enriched in hydrogen and carbon monoxide,
said process comprising:
a) introducing fuel into a lean exhaust gas stream to reduce its normalized air/fuel
ratio to a value of below 1 and conducting this exhaust gas stream over a reforming
catalyst;
b) burning a first portion of the injected fuel on the catalyst to heat the reforming
catalyst to a reforming temperature; and
c) converting a second portion of said injected fuel into a gas comprising carbon
monoxide and hydrogen.
10. The process according to claim 9, wherein the gas comprising carbon monoxide and hydrogen
is joined with a second exhaust gas stream to form a joined exhaust gas stream enriched
in carbon monoxide and hydrogen.
11. The process according to claim 10, wherein the joined exhaust gas stream is treated
in an exhaust gas cleaning unit before it is emitted into the atmosphere.
12. The process for regenerating a NOx storage catalyst comprising the process of claim
10 and exposing said joined exhaust gas enriched in hydrogen to the NOx storage catalyst.
13. The process for regenerating a particulate filter comprising the process of claim
10 and exposing said joined exhaust gas enriched in hydrogen to the particulate filter.
14. A process for generating electricity in a fuel cell wherein the gas comprising carbon
monoxide and hydrogen according to claim 9 is exposed as fuel to a fuel cell.
15. A method of using an apparatus for auto thermal steam reforming, said method comprises
operating a motor vehicle, wherein said motor vehicle comprises the apparatus of claim
1.